1. Field of the Invention
The present invention relates to an X-Y table. More particularly, the present invention relates to a belt drive system of a stage of the X-Y table, which produces few particles when used, and thus is particularly useful in a facility in which a clean environment is required.
2. Description of the Related Art
Generally speaking, microscopes are widely used in many fields to produce enlarged images of objects under study. Various kinds of microscopes are in fact employed in the field of semiconductor device fabrication to examine the fine wiring pattern that is often formed on the surface of a semiconductor wafer.
The fabrication of semiconductor devices involves highly precise processing steps. Wafers that do not meet certain threshold standards or which are inferiorly-manufactured are discarded to prevent the malfunctioning of the devices into which the wafers are to be incorporated. Otherwise, the yield of such devices would be low.
Therefore, various kinds of tests and analyses are performed on the wafers between the processing runs or within the course of one processing run. Among these tests, those for detecting contamination or a malfunction of the wafer are especially important in achieving a high yield of end products.
Such contamination or manufacturing defect can be directly detected by an operator, and the degree to which the wafer is contaminated or flawed can be observed using a microscope. To this end, the wafers in a production line are first examined with the naked eye. First, the operator removes the wafer from a cassette with a pair of vacuum tweezers, and loads the wafer onto a boat where it is held under suction. Then the operator illuminates the wafer. Even very small particles or dust can be detected this way by shining white light or monochromatic light of a short wavelength on the wafer at a certain angle. That is, light reflected at that angle from the particles or dust exposes the presence of the same rather clearly.
Contamination of or defects in the wafer, which are not capable of being detected by the above method, are checked for by using a metal microscope or an electron microscope.
FIG. 1 shows a conventional apparatus for inspecting wafers. In this apparatus, the wafer 1 is provided on an auto-loader 5, which can adjust the angle at which the wafer is held in order to provide the appropriate angle of reflection when the wafer is illuminated during the course of its visual inspection. If the wafer passes the inspection, the auto-loader 5 automatically moves the wafer to a microscope 2 so that the wafer 1 can be examined in more detail. In this respect, the microscope 2 has a pair of ocular lenses 4 and a set of objective lenses 3 for producing an enlarged image of the wafer 1.
The microscope 2 has an X-Y table 10 comprising a stage disposed beneath the objective lenses 3 of the microscope 2. The wafer 1 is secured on the stage by suction. The stage also allows the wafer 1 to be moved left and right and back and forth along X and Y axes perpendicular to one another so that the entire surface of the wafer 1 can be examined under the microscope 2.
The conventional X-Y table 10, as shown in FIG. 2 and FIG. 3, comprises: a wafer holder 11 which is rotatable and produces a vacuum to chuck the wafer transferred to the microscope 2 from the auto-loader 5; an X-axis moving plate 12 which is movable in the direction of the X-axis and has the wafer holder 11 mounted to its upper surface; a Y-axis moving plate 14 made up of two members disposed on opposite sides of the X-axis moving plate 12, and connecting rods 15 connecting the members together; a respective bearing 21 interposed between the X-axis moving plate 12 and each of the members of the Y-axis moving plate; and a fixed plate 16 which is fitted between the connecting rods 15, and supports the X-axis moving plate 12 and the connected members of the Y-axis moving plate 14. The Y-axis moving plate 14 and the bearings 21 guide the X-axis moving plate 12 for movement in the direction of the X-axis. On the other hand, the fixed plate 16 guides the Y-axis moving plate 14, along with the X-axis moving is plate 12 sandwiched between the connected members thereof, in the direction of the Y-axis. Specifically, the fixed plate 16 guides the connecting rods 15 of the Y-axis moving plate 14.
In addition, a joy stick 13 projects from one end of the X-axis moving plate 12. An operator can manipulate the joystick 13 to push and pull the X-axis moving plate 12 back and forth and to the right and left to quickly position the wafer 1. A double-control knob 24 extends through one of the connecting rods 15 of the Y-axis moving plate 14. The double-control knob 24 is used for moving the wafer 1 in fine increments along the X-axis and the Y-axis.
The double-control knob 24 includes an X-axis control knob 22 and a Y-axis control knob 23 integrated with an X-axis pinion 18 and a Y-axis pinion 20, respectively. An X-axis rack 17 fixed to the X-axis moving plate 12 meshes with the X-axis pinion 18. A Y-axis rack 19 integral with the fixed plate 16 meshes with the Y-axis pinion 20.
Therefore, rotating the X-axis control knob 22 and the Y-axis control knob 23 of the double control knob 24 moves the stage slowly in the direction of the X-axis and in the direction of the Y-axis, respectively. The gearing causes the wafer to move in fine increments along the X and Y axes.
However, the racks 17, 19 and pinions 18, 20 are meshing even during the relatively high-speed movement of the stage allowed for by the joystick 13. Accordingly, the gearing abrades, generating particles which often contaminate and thus adversely affect the wafer. Furthermore, under the heavy load of moving the stage frequently, the gearing becomes severely worn especially at the center portions of the racks. The increases in clearances between the racks 17, 19 and pinions 18, 20 as the result of such localized wear gives rise to a so-called "backlash phenomenon". The life of the machine components (gear elements) is shortened by the vibrations generated by the backlash.
In addition, if the fixed plate is not horizontal, the stage is inclined and the X-axis moving plate and the Y-axis slide downward along the incline when the joystick is released. Accordingly, it is difficult to focus the image when taking a photograph of the object.
Further, because the pressure applied by a rack to the pinion meshing therewith is concentrated locally on the pinion, the diameter of the pinion varies among the portions thereof meshing with different portions of the rack. As a result, rotating a knob of the double control knob over equal increments moves the wafer on the stage over non-uniform increments.
Finally, the physical engagement of the racks and pinions limits the speed at which the stage can be moved with the joystick.